This invention relates to power plants for generating electricity. More specifically, it pertains to a pressure control system for improving power plant performance by reducing air inventory and increasing overall heat transfer in the power plant's condenser and thus lowering back pressure to a low pressure turbine. The pressure control system is incorporated into the condenser's air removal system, which contains a two-stage liquid ring vacuum pump so that the operating pressure at the inlet of the vacuum pump can be controlled to optimize the condenser's performance.
A condenser is an essential but also largely neglected component in a power plant for generating electricity. In a 1977 report of the Electric Power Research Institute (EPRI), Anson estimates (1) that the loss of large fossil power plant availability directly attributable to condenser problems is 3.8%; and (2) condenser performance can significantly affect heat rate and generation capacity. In a 1988 EPRI report, Piskovsky, et al, estimate that a 3377 pascal (one inch of mercury) increase in back pressure could result in a 2% reduction in generation capacity. Also, excessive air or cooling water leakage in the condenser of fossil power plants can cause premature boiler tube failure and degradation of any component coming into contact with the condensate and feedwater, such as feedwater heaters and turbines. Such leakage is known to cause damage in steam generators for pressurized water reactors in the form of denting, stress corrosion cracking and corrosion fatigue. The cost to repair or replace damaged components and the financial loss as a result of the inability to produce power during an outage are substantial, for example, in a 1997 report of the Argonne National Laboratory, J. C. Van Kuiken, et. al, estimate that the replacement energy cost for a short term outage of 1000 megawatt nuclear power plant is in the range of several hundred thousand dollars per day.
The design of condensers has been based on an empirical, or black box, method without requiring any knowledge of distributions of velocity, pressure, temperature, air concentration, and condensation rate. The approach of condenser manufacturers has been to oversize heat transfer area. As it has long been known that excess air in the condenser degrades heat transfer and increases back pressure, an efficient air removal system is essential to condenser performance. There was very little theoretical work done for improving condenser design and essentially no attempt to optimize condenser performance. However, there are substantial economic benefits from improving condensers and recently the electric utility industry has paid increasing attention to improving condenser performance, design, and reliability. To improve condenser performance, design and reliability, it is necessary to understand detailed velocity, pressure, temperature, air concentration, and condensation rate distributions throughout a condenser, as well as to properly design an air removal system. Owing to significant advances in computer technology, detailed velocity, pressure, temperature, air concentration and condensation rate distributions can now be computed with codes such as COMMIX-PPC developed by Chien et al of Argonne National laboratory in 1997 and a scientific approach or method can be used to optimize condenser design including the condenser's air removal system, which is viewed as one of the boundary conditions for the condenser.
In most modem power plants, the air removal system typically includes an offtake pipe, or pipes, a two-stage, liquid ring vacuum pump (TSLRVP), a vacuum pump seal water heat exchanger, and a vent. Air, a non-condensable gas, mixed with steam pass through the air offtake pipe to the TSLRVP. Some power plants have an air removal system that uses an ejector. However, the performance of air removal systems with injectors under overload conditions for non-condensable gases are recognized as being inferior to the performance of air removal systems with a TSLRVP. As power plants are aging, more air leakage into condenser will occur due to foundation settling, vibration, thermal cycling and creep providing mechanisms for leak propagation . . . etc. The current trend in the power industry is to select an air removal system with a TSLRVP in new power plants and to replace an ejector by a TSLRVP in air removal systems in existing power plants.
To facilitate removal of air from a condenser, an air offtake pipe within a tube bundle should be located in the lowest pressure zone in the shell side of a condenser. If the air offtake pipe is not located in the lowest pressure zone in the shell side, air pockets will be formed in the tube bundles in the condenser leading to an increase in air inventory, a reduction in overall heat transfer, and thus to an increase in back pressure. This is evident by observing the operating pressure of a condenser, which in general is much higher than the corresponding pressure specified in the condenser performance curve. Reducing the size of air pockets or eliminating air pockets inside a condenser will decrease the air inventory, improve condenser performance and increase power plant efficiency. The logical approach for reducing air inventory is to lower pressure at the air offtake pipe to facilitate removal of air by the air removal system.
The prior art recognizes that spraying cold water into the inlet port of a TSLRVP condenses steam in the steam and air mixture from the condenser reduces the volume of gas mixture to be handled by the vacuum pump. This method is used to increase the capacity of the vacuum pump as stated in Bulletin No. 795-B of the NASH Engineering Co. 1988 and is not used to decrease pressure at the inlet of TSLRVP. Audouin in U.S. Pat. No. 1,372,926 uses an auxiliary condenser to remove a large amount of steam from a condenser to reduce its operating pressure. However, the auxiliary condenser is relatively large and requires an additional pump. The instant invention reduces the operating pressure of a condenser by facilitating removal of air from the condenser, rather than by removing a large amount of steam from the condenser.